Chop fall foliage to add to compost

PASCO, Wash. -- Autumn has arrived. What are we going to do with all the falling leaves?

How about composting them?

Composting is a great way to recycle leaves and other garden waste, but beginning and experienced composters may have some questions about which leaves can and can't be composted.

Here are some of the frequently asked questions that come across my desk.

Q: Can I compost pine needles? Will it make my soil too acid like my gardening books indicate?

A: This is a persistent myth. Pine and other conifer needles won't make your soil too acid. Pine needles are very acidic before they start to break down, but once composted they become slightly acid to neutral along with the rest of the composted materials. University research reveals that most compost has a pH of 6.8 (slightly acid) to 7.0 (neutral).

One concern with composting pine needles is that they're high in plant waxes and lignin, making them slow to decompose. If you want to use pine needles or other conifer foliage in a compost pile, chop them up well before adding them to the pile. Don't add more than 10 percent by volume because they're so slow to break down.

Q: I heard that you shouldn't compost oak leaves. Is this true?

A: No, this isn't true either. I'm not sure how this myth originated, but oak leaves cause no problems in a compost pile. However, they are high in lignin (a complex organic compound that binds wood fibers together) and slower decomposing than most other tree leaves. Again, chop them up well and don't overload the compost with them.

Q: Someone warned me against composting sycamore leaves. Why shouldn't I compost them?

A: One problem with composting sycamore leaves is that there tends to be lots of big leaves, sometimes too many for a simple backyard compost. I recommend chopping up sycamore leaves first using a shredder or by mowing over them. By chopping them up, you create more surface area where the decay organisms can work, plus it reduces the volume you have for adding to a compost pile. When working with sycamore leaves, your nose and throat may become irritated from the pubescence or fuzz on the undersides of the leaves, so you may want to wear a dust mask.

Q: Is it true that walnut leaves are toxic and shouldn't be used in compost?

A: It is true that black walnut roots produce a plant chemical called juglone that is toxic to certain plants, such as tomatoes and rhododendrons, when they're growing in proximity to the tree roots. Black walnut sawdust, wood chips, nut hulls, bark and leaves contain juglone. However, the leaves can be safely used in compost piles because the juglone breaks down during the decay process. Persian (English) walnuts also produce juglone but in much smaller quantities, and they aren't considered "toxic" unless they're grafted plants with black walnut rootstocks.

* Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

This story was originally published October 5, 2012 at 12:00 AM with the headline "Chop fall foliage to add to compost ."